Cloud response and feedback processes in mixed-phase clouds perturbed by ship exhaust

As the Arctic sea ice continues to melt in a warmer climate, commercial shipping is projected to increase by the mid-century due to newly opened ice-free passage ways north of the Arctic Circle. The impact of ship exhaust on frequently occurring Arctic low clouds, which can form under very clean conditions, remains largely unknown. Satellite retrievals suggest that the Arctic is covered 60 - 90 % of the time by clouds below 3 km and that most of the clouds are single-layered mixed-phase clouds. For mixed-phase clouds to be maintained for several days, a delicate equilibrium between the liquid and the ice phase needs to be established. As the ship exhaust is mixed into the cloud layer, this equilibrium state is perturbed locally by hydrophilic aerosol concentrations reaching a hundred times their background concentration. Hence, cloud condensation nuclei (CCN) concentrations may be enhanced locally. Furthermore, measurements of exhaust plumes near Gothenburg (Sweden) suggest that in addition to CCN perturbations, ships may also enhance ice nucleating particle (INP) concentrations by a factor 2-10.In this study we analyze possible feedback processes occurring in mixed-phase clouds perturbed by ship exhaust using large-eddy simulations. Simulations of a clean single-layered mixed-phase cloud are constrained by measurements from the mixed-phase cloud experiment campaign (M-PACE) obtained during October 2004. The simulations are then seeded with increasingly higher hydrophilic aerosol (50 - 15'000 cm-3) and INP (0 - 5 l-1) concentrations.Our results show that the cloud response is largely dependent on the background ice crystal number concentration. Mixed-phase clouds containing few ice crystals, Ο(0.1 l-1), largely respond like warm-phase stratocumuli: as the cloud is seeded by CCN more numerous and smaller cloud droplets are activated (Twomey effect), the liquid precipitation is shut off and the liquid water path is increased (rapid adjustment). However, mixed-phase clouds with high background ice crystal number concentrations, Ο(1.0 l-1), show a much stronger response in the ice than in the liquid phase when seeded with CCN. In these clouds the cloud feedback is largely constrained by the cloud top cooling rate, which is increased in perturbed clouds and leads to increased immersion freezing rates.